This invention relates to a method and apparatus for non-destructive materials testing using millimeter-wave detection of thermal waves, and more particularly, to a method and apparatus for generating thermal waves in electrically nonconducting materials and for identifying thermal inhomogeneities (including physical defects) at subsurface levels using millimeter-wave radiometry.
It is well known in the prior art that periodic heating of a sample using a beam emanating from an intensity modulated source such as a laser will generate thermal waves. Thermal waves will propagate from the heated spot, and will interact with thermal boundaries and barriers in a manner that is mathematically equivalent to the scattering and reflection of conventional propagating waves. Features on or beneath the surface of the sample that have thermal characteristics different from their surroundings will reflect and scatter thermal waves so that variations in thermal characteristics will be revealed by imaging of the thermal waves.
Variations in thermal characteristics such as density, specific heat, and, most important, thermal conductivity, arise from variations in the local lattice structure of the material, and may not be detectable with conventional optical or acoustic probes. Other features affecting thermal waves may result from changes in basic material composition or the presence of mechanical defects, such as cracks, voids, or delaminations. Many of these features can be imaged by optical, x-ray, or acoustic probes, but thermal-wave imaging may offer advantages.
Detection and imaging of thermal waves is currently accomplished by several different techniques through the effect of the thermal waves on the temperature on the surface of the sample using gas cell, optical beam deflection, or infrared radiation emitted from the sample, or through their generation of thermoelastic signals in the bulk of the sample using piezoelectric techniques. (See further, A. Rosencwaig, "Thermal-Wave Imaging," Science, Vol. 218, pp. 223-228, 1982, and U.S. Pat. No. 4,578,584 issued Mar. 25, 1986, to Baumann, et al.)
Difficulties in the imaging of thermal waves arise primarily from the fact that thermal waves are heavily damped and generally can travel only one or two wavelengths before becoming too weak to detect. Imaging is especially difficult in poor thermal conductors such as most ceramics where typical thermal diffusion lengths are only about 0.2 mm at a modulation frequency of 1 Hz, which compares to a diffusion length in a good thermal conductor like aluminum of 5.6 mm at 1 Hz. Therefore, defects below the surface of a thick, nonconducting sample are mostly inaccessible to prior art thermal wave imaging techniques which measure surface characteristics such as temperature.
Some prior art methods are limited in their application as well because they require physical contact between the detector and the sample (as in, for example, the use of gas-microphone or piezoelectric techniques). Detection methods involving low signal to noise ratios, or focusing on very narrow beam spots, require long integration and scanning times. Also, techniques involving infrared detection of radiation are subject to variations due to surface emissivity caused, for example, by surface irregularities, especially roughness.
It is well known in the prior art that microwave or millimeter-wave radiometry may be used to map thermal radiation and the application of microwave radiometry to medical diagnosis has provided evidence that microwave radiation at optimal wavelengths can penetrate subsurface to provide subsurface spatial resolution and to detect changes of temperature with good sensitivity. (See further, Barrett et al, "Detection of breast cancer by microwave radiometry," Radio Science, Vol. 12, pp. 167-171, 1977.) The present invention departs from the prior art in providing a method and apparatus for both the generation of thermal waves as well as the detection and imaging of thermal waves and the thermal features they reveal. In this sense, it is an active radiometric technique.
The use of millimeter-wave radiometry for materials testing offers several advantages. Because the millimeter waves can travel through electrically nonconducting materials without much attenuation, the radiations emitted by the entire thermal-wave swept volume of material can be detected, thus enabling deep subsurface features to be imaged. The sensitivity or minimum detectable temperature of a radiometer depends on various factors including the background antenna noise, receiver noise figure, predetection bandwidth, and post-detection integration time. With modern millimeter-wave components, sensitivities on the order of a milliKelvin are possible, and can be further improved by using cryogenic receivers and/or using the latest high-temperature superconducting technology for waveguide and receiver components. High signal-to-noise ratio and spatial discrimination are possible by using a focusing lens in front of an antenna horn, or by employing a dual radiometer as a phase-switched interferometer, or by changing the antenna inclination and receiving the millimeter-wave radiation at different angles. The radiometer itself is a compact system since its components including the antenna are small in the millimeter-wave range.
It is therefore a primary object of this invention to provide a method and apparatus for generating thermal waves in a sample and for measuring thermal inhomogeneities at subsurface levels using millimeter-wave radiometry.
In the accomplishment of the foregoing object, it is another important object of this invention to provide a method for measuring thermal waves which is only minimally dependent on surface emissivity.
It is another important object of this invention to provide a method for measuring thermal waves which provides a high degree of subsurface imaging capability and spatial resolution.
It is a further object of this invention to provide a full-field thermal wave imaging system which expedites the imaging process by eliminating the need to scan the object with a point-by-point technique.
It is a further object of this invention to present a system for thermal wave characterization of materials which is entirely noncontacting.
It is a further object of this invention to present an apparatus for measuring thermal waves which is compact and easy to use.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.